The long range goal of this project is to understand the role of programmed cell death (PCD) in the response of normal and tumor tissues to ionizing radiation. Radiobiological studies have historically focused on the dividing population of cells in tissues, the clonogens or stem cells, and much less is known concerning the response of the non-dividing terminally differentiated cells. However, it has been observed that certain tissues, e.g. salivary gland, respond to radiation in a dramatic way by initiating an acute response within a few hours of irradiation. This type of response is characteristic of PCD and although research on PCD is active in other areas of biology relatively little has been done to understand its role in irradiated cells. Thus, the specific aims of this project are designed to quantitate the magnitude of PCD in a number of important cells and tissues following irradiation as well as investigate the details of PCD pathways at the biochemical and molecular level. In specific aim 1, we propose to use rat thymocytes as a model system for ascertaining what radiation lesion in the cell initiates the PCD response. In addition, although it is known that messenger RNA and protein synthesis is required for PCD the identities of the specific genes and respective gene-products are not known. Therefore, another aspect of this aim is use molecular biological approaches to identify these genes and proteins. Specific aim 2 proposes experiments to extend Some of the observations made under aim 1 to a more radiobiologically relevant tissue, lithe salivary gland. This will primarily involve an analysis of the DNA fragmentation patterns characteristic of PCD in irradiated salivary tissue from rats and monkeys. Finally, in specific aim 3, we want to determine whether PCD plays a significant role in irradiated tumors. Several mouse tumors will be evaluated and the level of PCD induced will be correlated with tumor growth delay. The results from this research could lead to improvements in human cancer radiotherapy by providing an understanding of the various biochemical pathways involved in PCD which could in turn lead to the identification of inhibitors of PCD in irradiated normal tissues and enhancers of radiation-induced PCD in tumors.